This invention relates to web severing devices for continuously moving webs and has particular reference to a severing device for a paper web or webs passing through a printing press.
Web severing devices have been used heretofore in conjunction with sensing devices which sense a continuously traveling web passing through a printing unit and which activate the severing device in the event of a break or tear in the web. Such severing devices usually sever the web at a point prior to entry of the web into the printing unit to prevent wrap-up of the leading end of the torn portion of the web around various cylinders, rollers or guides to the extent that it an damage the press or its components.
In large printing plants, such as are used in the newspaper printing art, a number of printing units are provided to print on one or both sides of several webs, and such webs are fed in superposed relation through a folder unit where they are passed over various formers, folding rollers, cutting and folding cylinders, etc. to form the finished newspaper or similar product. As the webs pass through the various printing units and into the folder unit, they generally are fed over several power driven feed rollers, such as drag rollers, forming rollers, nip rollers, etc. which are driven at progressively higher peripheral speeds as the webs approach the folder unit so as to apply a progressive gain or tension to each web for the purpose of keeping the same taut as it travels through the folder unit. This prevents wrinkling of the paper and also enables accurate registration between the several webs. However, if a break occurs in a web as it passes through a printing unit or at any point intermediate the printing unit and the folder unit, the sudden release in tension in the web causes a wave or surge to travel along the length of the web and through the folder unit. Such surge tends to change the registration of the torn web relative to the remaining webs and also tends to wrinkle or bunch such torn web at points where it passes between certain cylinders or rollers in the folder unit, causing jamming and other damage before the press can be stopped. Also, considerable time is generally consumed in removing the jammed web preparatory to rethreading the same through the folder unit.